This invention relates to a method of separating isotopes of an element, which comprises bringing a gaseous compound of the element into a condition in which the molecules of the compound are in one well-defined vibrational state, and selectively exciting the molecules containing the pre-determined isotope by means of a source of radiation, and further exciting, and/or directly dissociating, said selectively excited molecules by means of at least a second source of radiation, using at least two laser beams which are converted using so-called "four-wave mixing" in a Raman cell. The invention further relates to apparatus for the application of the method. A method and apparatus of this kind are disclosed in the article in I1 Nuovo Cimento, Vol. 6D, N. 6, Dec. 1985, pp 559-566. More specifically the article describes how, by means of "four-wave mixing" the efficiency of the conversion of the laser beam necessary for the selective excitation of molecules of uranium hexafluoride containing the isotope, U.sup.235, is increased. An increase of the efficiency of conversion leads to an increase of the efficiency of the enrichment of uranium.
The method of enriching uranium described in the above article can be briefly explained further as follows.
The molecules of the gas, uranium hexafluoride, consist as to 0.7% of .sup.235 UF.sub.6 and as to 99.3% of 238UF.sub.6. Owing to the difference in mass between the two isotopes, there is a difference in frequency of internal vibration modi in molecules containing either the one or the other isotope; this is a so-called isotope shift. Now, by shining (laser) light of a suitable wavelength on to the gas, it is possible to selectively excite molecules containing one isotope (in practice always .sup.235 UF.sub.6) into a vibration mode whose frequency exactly matches that of the laser light, whereas molecules containing the other isotope are excited with a much lower probability, if at all. The wavelength suitable therefor is in the infrared at about 15.92 .mu.m. The selectively excited molecules should subsequently be removed from the gas or the gas stream in which they are contained, before there is actual separation or enrichment. This is effected in a next step or steps, in which the molecules are excited further to such a high vibration mode that the molecule becomes unstable and disintegrates or dissociates. Further excitation can again be effected by means of laser radiation.
In the prior method, and also in a comparable prior method described in an article in Optics Letters, Vol. 7, No 5, pp 212-214, the first source of radiation needed to effect the first selective excitation of the UF.sub.6 is a so-called Raman shifter. This is a cell filled with, in this case, hydrogen, into which the light from a CO.sub.2 laser (.lambda.=10.178 .mu.m) is sent. Through a non-linear interaction with the material system (here the hydrogen gas, therefore), the light from the CO.sub.2 laser is converted into light having the desired wavelength of 15.92 .mu.m. This process is called Stimulated Raman Scattering. The frequency of the 10 .mu.m and 16 .mu.m beams are related to each other as follows EQU .upsilon..sub.10 -.upsilon..sub.16 =.upsilon..sub.0-2
(1)
In this equation, .upsilon..sub.0-2 is the frequency of a rotational transition in the hydrogen molecule which takes place during the scattering process. This rotational transition of the molecule, characterized by rotational quantum numbers of 0 and 2, has a given fixed frequency (354.33 cm.sup.-1) and by virtue of relation (1) this fixes the difference in frequency between incoming and outgoing radiation. What happens, therefore, is that the incoming radiation re-emerges from the cell shifted in frequency, which accounts for the name Raman shifter. It is further noted that, in order to achieve Raman conversion, a certain minimum intensity is required for the pump beam .upsilon..sub.1. This depends on the scattering medium, the specific transition in the medium, and the incident frequency .upsilon..sub.1. A term often used in the literature in this connection is a conversion threshold value. For the relatively long wavelength of the CO.sub.2 light, the conversion process turns out to be rather weak, i.e., a very high intensity beam of CO.sub.2 light must be shone. into the Raman cell for there to be any degree of conversion. For an industrial process, where the non-converted light from the CO.sub.2 beam can be regarded as waste, it is of course extremely important, from the point of view of efficiency, to achieve as high a conversion as possible. This means that the aim should be for a method which makes the overall efficiency of the enrichment process as high as possible. In the prior method, the efficiency is increased by using so called "four-wave mixing". As the name implies, this involves 4 waves, say with frequencies of .upsilon..sub.1, .upsilon..sub.1s, .upsilon..sub.2, .upsilon..sub.2s. In this case, .upsilon..sub.1, .upsilon..sub.2 mean the frequencies of two CO.sub.2 light beams at 10.178 and 10.2 .mu.m, respectively, and .upsilon..sub.1s, .upsilon..sub.2s mean the appurtenent Raman-shifted frequencies: EQU .upsilon..sub.1 -.upsilon..sub.1s =.upsilon..sub.2 -.upsilon..sub.2s =.upsilon..sub.0-2
(2)
In the present case, beam 1 (frequency .upsilon..sub.1) has such a low intensity that there is hardly, if at all, any Raman conversion, for the high intensity required for conversion can hardly, if at all be achieved, because rather high requirements must be imposed upon the frequency .upsilon..sub.1s (for the selective excitation) and hence upon the frequency .upsilon..sub.1 of the CO.sub.2 laser, (which could mean, for example, that a large number of optical elements, each by itself giving a loss, must be disposed in the CO.sub.2 laser oscillation cavity). If now, together with beam 1 beam 2 is shone into the cell (i.e., overlapping in both space and time), and this second beam does have an intensity sufficiently high for Raman conversion, then, owing to the four-wave mixing process, beam 1 can begin to convert as well. The efficiency with which beam 1 is converted is virtually equal to that of beam 2. The resulting beams (frequencies .upsilon..sub.1s, .upsilon..sub.2s) are thereafter used (or selective first excitation (.upsilon..sub.1s) and subsequent excitation (.upsilon..sub.2s). As no stringent requirements are imposed upon .upsilon..sub.2, a relatively simple CO.sub.2 laser, e.g., of the TEA type can be used here.